Thin-layer chromatography of aliphatic 2,4-dinitrophenylhydrazones

Formation of methyl ketones during ripening of Cheddar cheese

Cheddar cheeses made using single strains AM2and HP ofStreptococcus cremorisas starter were analysed for methyl ketones during ripening at 7°C. The level of odd-numbered C3–C15methyl ketones increased very slowly in all cheeses and at maturity represented only 15–20% of the level potentially available from the degradation of esterified β-ketoalkanoic acids of milk fat in cheese. The potential for formation of the methyl ketones in unripened cheese curd was similar to the sum of free and potentially available ketones in mature cheese which suggests that the oddnumbered ketones once formed are not reduced or otherwise degraded by the normal cheese microflora. Significant variations were observed in the level of 2-butanone, which, in some cases, far exceeded the quantity which could be formed by breakdown of β-ketovaleric acid. Organoleptic evaluations and comparison of the levels of methyl ketones present in mature cheese with published threshold data suggest that while the methyl ketones may contribute to flavour they do not in their own right impart a typical mature Cheddar flavour to cheese.

Flavour defects in edible casein and skim-milk powder: II. The role of aliphatic monocarbonyl compounds

Aliphatic monocarbonyls have been quantitatively estimated in casein and skim-milk powder during storage. Concurrently these products have been evaluated organoleptically. A close relationship was evident between monocarbonyl level and the flavour or age of the lactic casein. Furthermore, the greater flavour stability of rennet casein and skim-milk powder was coupled with reduced monocarbonyl content. Fractionation of monocarbonyls into classes and individual compounds has shown that the higher levels in lactic casein compared with other low-fat products can be accounted for by a high level of acetone. The absence of appreciable quantities of other odd-numbered methyl ketones suggests that either acetone is derived from a non-lipid constituent of acid casein or decarboxylation of acetoacetate is specifically catalysed at acid pH. All methyl ketones identified in these low-fat dairy products were present in subthreshold concentrations. Saturated and unsaturated aldehydes were present at similar levels in stored lactic casein, rennet casein and skim-milk powder samples irrespective of their degree of musty or stale flavour.

Development of an enzymatic method to quantify methyl ketones from bacterial origin

Methyl ketones are detected in dry fermented sausages in which they contribute to the cured aroma. They have been associated with the inoculation of Staphylococcus carnosus used as starter culture. To evaluate the ability of bacterial starters to produce methyl ketones it was necessary to develop a rapid method. The method consists of a reaction catalyzed by a commercial NADPH-dependent alcohol dehydrogenase that reduces the 2-pentanone to its secondary alcohol. The linearity, the specificity, and the robustness were studied. Its accuracy was confirmed by comparison with the gas chromatography technique. Finally, the method was validated on biological samples such as the 2-pentanone produced by Staphylococcus carnosus. The enzymatic method offers some advantages over the gas chromatography, as it is faster, simpler, and inexpensive, guaranteeing an effective way to assess bacterial ketone production.

The composition of the scent of the green vegetable bug, Nezara viridula

By a combination of gas, paper and thin-layer chromatography, and infra-red, visible, ultra-violet, mass and nuclear magnetic resonance spectrometry, twenty components were detected and investigated in the secretion collected directly from the scent storage reservoir of Nezara viridula . Structures were suggested for eighteen compounds, which normally comprised over 99.9 % of the scent. Three previously identified compounds were confirmed as trans -hex-2-enal, trans - dec-2-enal and n -tridecane. Other major components (1% or more) identified were 4-keto- trans -hex-2-enal, trans -hex-2-enyl acetate, trans -dec-2-enyl acetate and n -dodecane. Trace amounts were found of trans -prop-2-enal, trans -but-2-enal, trans -oct-2-enal, cis -dec-2-enal, 4-keto- trans -oct-2-enal, trans - oct-2-enyl acetate, methyl ethyl ketone, ethyl propyl ketone, methyl heptyl keton, n undecane, and (probably) 4-keto-hex-2-ene. The distribution of the components between the two phases of the scent was studied. The analytical results are discussed in terms of identified constituents of scents from other bugs.